The surface properties of substrates used in applications relating to integrated circuits, electronic packages, and printed circuit boards are commonly modified by plasma treatment. In particular, plasma treatment is used in electronics packaging, for example, to increase surface activation and/or surface cleanliness for eliminating delamination and bond failures, improving wire bond strength, ensuring void free underfilling of chips on circuit boards, removing oxides, enhancing die attach, and improving adhesion for die encapsulation. Typically, one or more substrates are placed in a plasma treatment system and at least one surface of each substrate is exposed to the plasma. The outermost atomic layers may be removed from the surface by physical sputtering, chemically-assisted sputtering, chemical reactions promoted by reactive plasma species, and combinations of these mechanisms. The physical or chemical action may also be used to condition the surface to improve properties such as adhesion or to clean undesired contaminants from the substrate surface.
During semiconductor manufacture, semiconductor die are commonly electrically coupled by wire bonds with leads on a metal carrier, such as a lead frame. Lead frames generally include a number of pads each having exposed leads used to electrically couple a single semiconductor die with a circuit board. One semiconductor die is attached to each pad and external electrical contacts of the die are wire bonded with nearby portions of the leads.
Each semiconductor die and its wire bonds are encapsulated inside a package consisting of a molded polymer body designed to protect the semiconductor die and wire bonds from the adverse environment encountered during handling, storage and manufacturing processes as well as to dissipate the heat generated from the semiconductor die during operation. A common molding material used to fabricate such packages is an epoxy resin filled with silica or silicon particles.
During the molding process, the lead frame and the multiple attached semiconductor die are positioned between two mold halves. One mold half includes numerous cavities each receiving one of the semiconductor die and defining the package shape. The mold halves are pressed together in an attempt to seal the entrance mouths to the cavities. The molding material, which is injected into the mold, fills the open space inside the cavities for encapsulating the semiconductor die and wire bonds. However, molding material can seep out of the cavities between the mold halves and form thin layers or flash on the exposed leads. This thin flash has a thickness typically less than about 10 microns. Flash is detrimental as it may affect the ability to make high quality electrical connections with the encapsulated semiconductor die.
Flash may be prevented during the molding process by covering the backside of the lead frame with tape. However, adhesive may be transferred from the tape to the lead frame backside and remain as a residue after the tape is removed. In addition, tapes suitable for this application are relatively expensive, which adds to the cost of manufacture. Flash may be removed after molding by mechanical and chemical techniques, or with a laser. These removal approaches also suffer from deficiencies that restrict their use. For example, the lead frame is susceptible to damage from mechanical flash removal techniques, such as chemical mechanical polishing. Chemical processes may be ineffective unless highly corrosive chemicals are used, which potentially raises issues of worker safety and waste disposal of the spent corrosive chemicals. Laser removal is expensive and leaves a residual carbon residue behind on the lead frame.
There is thus a need for a plasma treatment process that can efficiently and effectively remove excess molding material from a substrate.